The invention relates to a fireproof mixture, in particular on the basis of magnesia, as well as an elastifier therefor and a method for producing the mixture and a moulded body thereof.
Hereinafter, the carrier of the refractory quality and thus customarily also the main component of a fireproof moulded body or of fireproof masses is termed a resistor. This resistor can, in the most general case, be a metallic oxide, mineral, fireproof substance such as MgO, Al2O3, doloma or similar.
Hereinafter, the term elastifier is used to designate minerals which lead to an increase of the thermal fatigue resistance of a mixture of resistor and elastifier—as opposed to a pure resistor—as a result of an inherent, relatively high refractory quality, but an extension under temperature different from the resistor, through formation of micro-fissures and further effects.
Fireproof moulded bodies, in particular basic fireproof materials on the basis of magnesia, doloma, chromite and/or spinel (MgAl2O4), are used in all high-temperature processes with basic slag attacks, such as cement, lime, dolomite, iron and steel production and also in production of nonferrous metals and in the glass industry as lining materials for furnaces, vessels and treatment systems. With high refractory quality and good chemical resistances, the materials and moulded bodies are however highly brittle, i.e. have a high module of elasticity, thus resulting in negative influences on the service life with regard to the heat extension, stresses, mechanical load and the thermal fatigue resistance.
In addition, it is known that fireproof moulded bodies are also produced on the basis of Al2O3, with in particular bauxite, tabular oxide of aluminium or fused corundum being used as the raw material. The main fields of application for such stones are electrical furnace lids and kettles in the steel industry as well as cement kilns and furnaces in the glass industry.
It is known that the high thermal expansion stresses of basic fireproof products or moulded bodies are reduced by laying the fireproof stones with a mortar joint, metallic inserts such as sheets perforated sheets or nets arranged between them.
Further, numerous measures have been taken in the past in order to improve the thermal fatigue resistance, in particular of basic fireproof materials. Harders/Kienow, Feuerfestkunde, Herstellung, Eigenschaften und Verwendung feuerfester Baustoffe, Springer Verlag, 1960, Chapter 5.5, pages 754 and 755, states that the thermal fatigue resistance can be distinctly improved by the addition of chrome ore (magnesium chromate) and by a so-called mixture gap, i.e. minimisation of the share of medium grains (0.2 to 0.6 mm). However, a decisive disadvantage of the mixture gap is, on the one hand, that its effect is only sufficiently high in combination with a thermal fatigue resistance component such as magnesia or chrome ore in magnesia chrome stones if, on the other hand, no optimum grain packing density can be achieved in use of the mixture gap, as is required to achieve a high infiltration resistance against slags. Further, the quantity of chrome ore and the optimum grain fraction of the chrome ores has been defined with a view to the addition of chrome ore (e.g. Harders/Kienow, page 754). In order to achieve a satisfactory thermal fatigue resistance, quantities of chrome ore between 15 and 30% by weight have been recognised as being suitable. The elastifying effect of the chrome ore in moulded bodies on the basis of magnesia has been unsurpassed up to now. Decisive disadvantages of the use of chrome ore as an elastifier (thermal fatigue resistance component) are however that material fatigue takes place in a change of the furnace atmosphere and that the trivalent chrome oxide in the chrome ore is converted into toxic hexavalent chrome oxide by oxidation under the effect of alkalis, with all the problems connected with this from a work-hygiene and disposal point of view.
Attempts were made at an early stage (AT-PS 158208) to add aluminium oxide, corundum and aluminium powder to magnesia stones in order to improve the thermal fatigue resistance, with spinel (MgAl3O4) being formed when the stones are burnt in situ. The spinel formed in this way is concentrated in the matrix, which means that the matrix decisive for the strength is preferably destroyed in the attack of such stones by slags. In addition, the improvement of the thermal fatigue resistance which can be achieved is limited, as the share of Al2O3 necessary for a decisive improvement would have to be way above 8% by weight. However, this is not possible due to the excessive growth of the stones as a result of an increase in volume in the matrix, as otherwise dimensional accuracy and mechanical strength become too low and the porosity too high. A considerable improvement of both the heating fatigue resistance and also the chemical resistance of magnesia stones was only achieved by the addition of pre-synthesised magnesium aluminium spinel in the form of sintered or fused spinel, with the customary added quantities being between 15 and 25% by weight.
DE 41 19 251 A1 manifests that a spinel clinker of the magnesium oxide/aluminium oxide type, containing Fe2O3 and TiO2 on the borders between the crystal grains, is used in a chamotte slab.
Further, DE 44 03 869 manifests a fireproof ceramic mixture essentially containing MgO sinter as the carrier of the refractory quality, with a spinel of the hercynite type being used as an elastifier.
The thing common to all the attempts to replace the chrome ore with its outstanding elastification property for in particular basic fireproof products by materials with less reservations with regard to environmental hygiene is that, although elastification effects can be achieved, they are inferior to those of the chrome ore. A further disadvantage is that the elastifiers used, such as hercynite, fused or sintered spinel or molten zirconium oxide are synthetic raw materials which are considerably more expensive than the natural material chrome ore.
Fireproof moulded bodies and thus also the elastifying components are increasingly burdened in use, for example by greater thermal loads and thus increasing mechanical forming in industrial kilns (cement kilns, rotary lime kilns, steel casting ladles etc.) or by the increasing use of secondary combustion materials, which have a negative influence on the annexing property otherwise required, for example in rotary cement kilns, and lead to an undesired change of temperature with the accompanying destruction of the stones due to a reduced formation of annexing or flaking of annexing.